In a lithium ion secondary battery, a lithium-containing composite oxide (for example, LiMnO2) is used as a cathode active material. The lithium-containing composite oxide is a composite oxide containing, in addition to Li, transition metal elements of Ni, Co, Mn, and the like and other metal elements being doped elements as a composition. The crystal structure of the lithium-containing composite oxide is a layered structure having a metal atom layer, a lithium atom layer, and an oxygen atom layer.
The lithium ion secondary battery is required to have a characteristic in which a discharge capacity does not decrease by repetition of a charge and discharge cycle, (which will be called cycle characteristics, hereinafter). Further, in the lithium ion secondary battery, an improvement in characteristic in which a discharge capacity does not decrease on the occasion of high-rate discharge, (which will be called rate characteristics, hereinafter), is also required.
It is known that the cycle characteristics and the rate characteristics are affected by exchange of a metal atom and a lithium atom in the crystal structure of the lithium-containing composite oxide, (this phenomenon will be called cation mixing, hereinafter). This is thought that by cation mixing, a path where lithium ions should diffuse originally is blocked, and thereby a discharge capacity decreases, and as a result, the cycle characteristics of the lithium ion secondary battery decrease. Here, it is known that there is a negative correlation between the frequency of occurrence of cation mixing and a peak intensity ratio (I003/I104) between, in an X-ray diffraction pattern of the lithium-containing composite oxide, an integral value (I003) of a diffraction peak of the (003) plane and an integral value (I104) of a diffraction peak of the (104) plane. That is, it indicates that when the peak intensity ratio becomes large, the occurrence of cation mixing decreases.
Further, as a factor affecting the cycle characteristics, the crystallite diameter of the (110) plane of the lithium-containing composite oxide is known. Then, it is known that when the crystallite diameter of the (110) plane exceeds 100 nm, the cycle characteristics decrease.